util: introduce os_dupfd_cloexec() helper

Adapted from wayland's wl_os_dupfd_cloexec().

Suggested-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5369>

src/util/os_file.c

@@ -18,6 +18,11 @@
 #define O_CREAT _O_CREAT
 #define O_EXCL _O_EXCL
 #define O_WRONLY _O_WRONLY
+#else
+#include <unistd.h>
+#ifndef F_DUPFD_CLOEXEC
+#define F_DUPFD_CLOEXEC 1030
+#endif
 #endif
 
 
@@ -31,6 +36,50 @@ os_file_create_unique(const char *filename, int filemode)
 }
 
 
+#if DETECT_OS_WINDOWS
+int
+os_dupfd_cloexec(int fd)
+{
+   /*
+    * On Windows child processes don't inherit handles by default:
+    * https://devblogs.microsoft.com/oldnewthing/20111216-00/?p=8873
+    */
+   return dup(fd);
+}
+#else
+int
+os_dupfd_cloexec(int fd)
+{
+   int minfd = 3;
+   int newfd = fcntl(fd, F_DUPFD_CLOEXEC, minfd);
+
+   if (newfd >= 0)
+      return newfd;
+
+   if (errno != EINVAL)
+      return -1;
+
+   newfd = fcntl(fd, F_DUPFD, minfd);
+
+   if (newfd < 0)
+      return -1;
+
+   long flags = fcntl(newfd, F_GETFD);
+   if (flags == -1) {
+      close(newfd);
+      return -1;
+   }
+
+   if (fcntl(newfd, F_SETFD, flags | FD_CLOEXEC) == -1) {
+      close(newfd);
+      return -1;
+   }
+
+   return newfd;
+}
+#endif
+
+
 #if DETECT_OS_LINUX
 
 #include <fcntl.h>

src/util/os_file.h

@@ -24,6 +24,12 @@ extern "C" {
 FILE *
 os_file_create_unique(const char *filename, int filemode);
 
+/*
+ * Duplicate a file descriptor, making sure not to keep it open after an exec*()
+ */
+int
+os_dupfd_cloexec(int fd);
+
 /*
  * Read a file.
  * Returns a char* that the caller must free(), or NULL and sets errno.